Recovery of metal values



United States Patent Ofiflce 3,1133% Patented Dec. 10, 1963 3,113,860 RECGVERY F METAL VALUES Richard F. Pagel, Belleville, 111., assignor to American Zinc, Lead and Smelting Company, St. Louis, Mo., a corporation of Maine No Drawing. Filed Feb. 15, 1960, Ser. No. 8,508 4 Claims. (Cl. 75115) This invention relates to the recovery of metal values, and particularly nonferrous metal values, from ferrites. This application is a continuation-in-part of my application Serial No. 728,779, filed April 16, 1958, now abancloned.

The term ferrites as used herein, denotes ferric oxide complexes of the general form MO'Fe O in which M is a metal of the character of zinc, copper, cadmium, lead, and (ferrous) iron. While the method of this invention is applicable to ferrosoferric oxide (iron ferrite), under present conditions that application of the process is not commercially feasible, and for that reason, the present process is directed to the recovery of nonferrous metals. The method is described particularly as applied to the recovery of zinc, but it is to be understood that it is equally applicable to the recovery of such metals as copper and cadmium, and, in a slight modification, to the recovery of lead. To the extent that ferrites of nickel, cobalt and chromium are found, the process is applicable to them.

In the usual commercial process of electrowinning zinc, zinc oxide is dissolved in sulfuric acid. The resultant zinc sulfate solution, after purification, is brought into an electrolytic cell, Where the Zinc is deposited as metallic zinc, oxygen is liberated, and the sulfuric acid is regenerated. The spent electrolyte, which assays in the neighborhood of 185 grams per liter sulfuric acid, is used to dissolve more zinc oxide, and the cycle is repeated continuously.

The zinc oxide used in the process is obtained by roasting the zinc ore, generally a zinc sulfide. The ore contains varying amounts of iron, ranging from about 5% or even less, up to and higher. During the roasting process, a considerable amount of zinc ferrite is formed. This zinc ferrite is insoluble in the sulfuric acid solution used in the electrolytic process which has been described. Commonly the insoluble residue from the dissolving of the zinc oxide with the spent electrolyte may constitute 30% of the weight of the ore, and contain in the neighborhood of to 28% zinc and approximately 20-27% iron. Thus as much as 20% of the original zinc content is tied up in the residue. This problem has been apparent to the zinc industry for many years, but no satisfactory solution has been found.

It has been suggested that the residue be roasted with concentrated sulfuric acid to convert the zinc to zinc sulfate and make the desired metal values soluble. It has also been suggested that the residue be leached with concentrated sulfuric acid, in which the ferrites are soluble. These methods have not proved commercially feasible, because of the corrosion problems involved and the building up of sulfate ion in the electrolytic circuit. If the electrolytic plant is connected with a lead smelter, in which there are slag fuming facilities available, the residue may be dried and added to the molten slag bath. However, as a practical matter, the shipping and treatment charges involved in the latter process may make its use uneconomic.

One of the objects of this invention is to provide a process whereby ferrites may be dissolved in weak sulfuric acid.

Another of the objects of this invention is to provide a process whereby the greater portion of metals combined as ferrites may be recovered at an electrolytic plant, using 2 only the weak sulfuric acid available from the electrolysis.

Other objects will become apparent to those skilled in the art in the light of the following description.

In accordance with this invention, generally stated, a process is provided for recovering desired metal values from ferrites which are normally substantially insoluble in weak sulfuric acid solution, by treating the ferrites with weak sulfuric acid solution in the presence of metallic iron.

The residue from the leach section of an electrowinning plant is the solid component remaining after com pletion of the leaching operation. This residue is separated from the liquid components of the leach liquor before the latter is fed to the electrolytic cells. Such residues usually contain a large proportion of ferrite, and the invention will be described with reference to the recovery of metal values from such residues.

According to the present invention, such a residue is placed in a suitable vessel, such as a leaching tank, along with a weak solution of acid, such as spent electrolyte, in an amount at least stoichiornetrically sufiicient to dissolve the desired metal values contained in the ferrites. Metallic iron is then added. The amount of metallic iron may vary due to changing acid strength and the varying iron assay of the residue. The higher the iron concentration in the residue, the more metallic iron needed. The particulate solids in the mixture are dispersed in the liquid component thereof until visual inspection or assays show that the ferrite has been decomposed.

Dispersion of the particulate solids within the acid solution can be accomplished by comminuting the residue to such fineness that Brownian movement suspension occurs, or by agitation sufficient tokeep them from settling into a mass. Dispersion has the primary purpose of keeping the ferrite particles from settling into a mass thereby allowing more particle area to be exposed to the action of the acid in solution and thereby accelerating the rate at which these particles will be decomposed. Agitation of sufficient violence to maintain not only the particulate solids in the residue, but also larger pieces of iron in motion has particular advantage in cases where the solution contains metals below metallic iron in the electromotive force series, since these metals plate out on the iron, in the form of spongy deposits, which can be dislodged by agitation. If time is no consideration, neither agitation nor suspension is required for solutions which contain metals that are above metallic iron in the electromotive force series.

The temperature of the mixture is not critical, however, to facilitate the decomposition of the ferrite, the temperature of the mixture may be raised to near boiling.

In recovering those metals which are below metallic iron in the electromotive force series, such as copper, etc., care should be taken to ensure that all of the metallic iron present is dissolved and that sufficient time and excess acid be employed to dissolve the metallic sponge of the metals that have been thrown down by the iron. In recovering metals that are above metallic iron in the electromotive force series, it is not necessary that the iron be completely dissolved since these metals will not plate out on the metallic iron present in the system.

The process is substantially the same as applied to residue from the leaching sections of electrowinning plants of any of the other metals which appear in the form of ferrites. The concentration of sulfuric acid in the spent electrolyte in other electrowinning processes (besides zinc) varies, usually on the side of less concentration. In the electrowinning of copper, for example, the concentration of sulfuric acid is in the neighborhood of 50 grams per liter. Presently, in the processes used by the copper industry, there are no copper ferrites present during the electrowinning process. Copper ferrites, like other ferrites, are formed during the roasting process which prepares the ore for leaching, however, in the case of copper the roasted copper ore is first smelted to obtain a blistered copper which is then subjected to the electrowinning process. The copper ferrites are broken up in the smelting process and thus are not present in the electrowinning step. However, if copper was formed by leaching the roasted ore the process herein described would be applicable since the copper ferrites would then be present.

The concentration of sulfuric acid in the spent electrolyte in the electrowinning of cadmium, is around 120 grams per liter.

While, as a practical matter, it is highly desirable to use spent electrolyte, which has a concentration of sulfuric acid in the various electrowinning processes of somewhere between 50 and 270 grams per liter, this is really only a matter of convenience. The process of this invention is effective as a practical matter at concentrations of sulfuric acid as low as grams per liter. Lower concentrations require an impractically long leach period.

When the ferrites have been decomposed, the desired metal values may be recovered by any of these methods known to those skilled in the art for the recovery of such values. In the case of zinc, copper, nickel, cobalt, chromium, and cadmium, the metal value will be present in the form of the sulfate, in solution. In the case of lead, the metal value is insoluble in sulfuric acid, but the iron has been dissolved, so that the lead, free of the ferrite complex, can be recovered from the residue.

As an illustrative example of the method of this invention as applied to the residue from a zinc electrowinning process, two 500 g. portions of plant leach residue were taken. The plant leach residue assayed:

Zinc percent Lead do Cadmium do Copper do Iron do 23.3 Sulfide sulfur do 1.35 Silver oz. per ton 32.31 Gold do 0.04

Percent Zinc (total) 25.5 Iron 21.0 Soluble zinc 1.9 (The other elements were essentially unleached.)

The other 500 g. portion of the plant leach residue was dispersed in 3000 cc. return electrolyte, assaying 186 g. sulfuric acid per liter, from the electrowinning cell. To this dispersion was added 35.0 g. of iron turnings. The mixture was agitated for 8 hours at approximately 95 C., in the same way as the first portion. At the end of this period, there remained an acid concentration of 12.0 g. per liter. The remaining residue weighed 151 g., and assayed:

Zinc (total) percent Lead do Cadmium do Copper do Iron do Sulfide sulfur do Silver oz. per ton Soluble zinc do It can be seen that in the first portion, 22.4% of the leach residue zinc and 22.5% of the iron were dissolved by the hot return electrolyte.

In the second portion, to which metallic iron was added, 87.4% of the zinc in the leach residue, 86.8% of the cadmium, 93.0% of the iron and 75.0% of the copper were dissolved by the hot return electrolyte. It is believed that some of the iron and copper in the remaining residue are to be accounted for by the presence of metallic iron which had not dissolved, and upon which copper was deposited. If the sulfide sulfur is present as zinc sulfide, 8.97% of the zinc in the 151 grams of remaining residue is in that form, and is therefore not separable by this process, leaving only about 0.5% unaccounted for (11.7% total zinc, less 2.4% soluble zinc, less 8.9% zinc sulfide).

The plant leach residue above is a typical example of the residues taken from the leaching stage of the electrowinning plant process. Another example of a plant leach residue is as follows:

Percent Zinc 23.4 Lead 1.71 Cadmium 0.26 Copper 1.83 Iron M 26.7 Sulfide sulfur 1.6

The metallic iron added to the system may be in the form of iron filings or turnings. The metallic iron so used should preferably be, but not necessarily required to be, a size that is readily dispersible in the solution. This is mainly for the purpose of exposing more surface area of iron to the action of the solution in the same manner as described for the ferrite. The amount of metallic iron to be used in the system is based on the iron assay of the residue, however, a convenient method for general run of the mill residues is that an amount of metallic iron equivalent to about 10% of the weight of the residue has been found satisfactory. However, in the case of solutions containing those metals above iron in the electromotive force series, the added iron can be in any form of larger pieces.

It might appear that the introduction of iron sulfate (ferric or ferrous) to the electrolyte would accomplish the same object as the dissolution of the metallic iron, but this is not the case. It might also appear that the dissolution of the ferrites might be accomplished by the use of other metals than iron, which would produce nascent hydrogen, in analogy to the known processes of roasting ferrites at dull red heat in the presence of hydrogen or other reducing gases. Again, however, this is not the case, the addition of such metals as zinc, magnesium, manganese, aluminum and copper, having no appreciable effect.

The term weak sulfuric acid, as used in the claims, means a sulfuric acid solution in which the ferrite is substantially insoluble. The concentration of sulfuric acid in such a solution is generally less than about 250 grams per liter. At sulfuric acid concentrations somewhere in the neighborhood of 200 to 250 grams per liter, the sulfuric acid begins to attack the ferrites directly, in hot solution, and as the concentration of sulfuric acid increases above 200 to 250 grams per liter, metallic iron begins to lose its purpose and may, at higher concentrations of hot sulfuric acid solution, be selectively attacked by the sulfuric acid, thus becoming a liability to the dissolving process. There is, of course, a relation between temperature and rate of dissolving of the ferrite at a given concentration of sulfuric acid, so that the term weak sulfuric acid is more accurately defined as a sulfuric acid solution in which the ferrite is substantially insoluble, than as a solution of a particular absolute concentration.

Thus, it can be seen that a process is provided which is simple, requires no concentrated acid nor any other extraneous reagent besides metallic iron, and which is effective to recover large amounts of metal values heretofore considered uneconomic to recover.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:

1. The process of recovering zinc values from a zinc ferrite containing residue from the dissolving of roasted Zinc ore by return electrolytic sulfuric acid solution in the electrowinning of zinc, comprising suspending said residue in return electrolytic sulfuric acid solution, in the presence of metallic iron, heating the mixture and agitating the mixture until the ferrite is dissolved.

2. In the art of recovering metal values from a metallic ore containing iron and other metals, the method comprising roasting the said ore to form ferrites, treating said ferrites with weak sulfuric acid solution in the presence of metallic iron until the said ferrites are dissolved, said acid solution containing not more than about 250 grams of sulfuric acid per liter of solution.

3. In the art of recovering metal values from a metallic ore containing iron and metals below iron in electromotive force series, the method comprising roasting said ore to form ferrites, treating said ferrites with a weak sulfuric acid solution in the presence of metallic iron and agitating, the acid solution containing not more than about 250 grams sulfuric acid per liter of solution.

4. In the art of recovering zinc from a metallic ore containing iron in association with zinc, the method comprising roasting said ore to convert the zinc and iron to zinc ferrite, and treating the ferrite with a weak sulfuric acid solution in the presence of metallic iron, until the iron and zinc content of said ferrite is substantially dissolved, said treating solution containing not more than about 250 grams of sulfuric acid per liter.

References Cited in the file of this patent UNITED STATES PATENTS 1,417,153 Fairlie et al May 23, 1922 1,467,516 Tainton Sept. 11, 1923 1,477,478 Elton et al Dec. 11, 1923 1,825,949 Haas Oct. 6, 1931 1,834,960 Mitchell Dec. 8, 1931 2,416,216 Rau et al Feb. 18, 1947 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 113,860 December 1O 1963 Richard F. Pagel It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column 3, line '75 for "do" read percent Signed and sealed this 26th day of May 1964.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. THE PROCESS OF RECOVERING ZINC VALUES FROM A ZINC FERRITE CONTAINING RESIDUE FROM THE DISSOLVING OF ROASTED ZINC ORE BY RETURN ELECTROLYTIC SULFURIC ACID SOLUTION IN THE ELECTROWINNING OF ZINC, COMPRISING SUSPENDING SAID RESIDUE IN RETURN ELECTROLYTIC SULFURIC ACID SOLUTION, IN THE PRESENCE OF METALLIC IRON, HEATING THE MIXTURE AND AGITATING THE MIXTURE UNTIL THE FERRITE IS DISSOLVED. 